1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus, such as a CD (Compact Disk) player and a VD (Video Disk) player. More particularly, it relates to an improvement of a vibration restraining technique in a tracking unit by a drive control apparatus for an optical pickup.
2. Description of the Related Arts
In a drive control apparatus for an optical pickup, a rough seeking mode is set as an operation mode to access a desired information track on a CD, at a high speed in a CD player. In this rough seeking mode, an operation to control an acceleration and a deceleration of the slider is performed according to a distance from the present position of the optical pickup to the target track.
In the rough seeking mode, at the time of drive-controlling the optical pickup, the control is performed in such a state that only a slider servo-loop is closed while a tracking servo-loop is opened. The purpose of opening the tracking servo-loop, is to prevent the servo-control from being applied with respect to the track other than the target track when the optical pickup crosses through the tracks.
In the slider servo-loop, for example, a track number is counted on the basis of a tracking position error, which is detected and supplied thereto. On the other hand, a desired track number is set by a keyboard. When the counted track number reaches the set value of the track number, the target speed data of the tracking unit is computed on the basis of a speed value (or an access target) inputted by the key board. The target speed data is converted into a reference value of the tracking unit speed. On the other hand, the track crossing speed (which is equal to the sum of the slider speed and the relative speed of the tracking unit with respect to the slider including the vibrating component of the tracking unit), is calculated on the basis of the tracking position error, so that an output value is obtained by subtracting the track crossing speed from the reference value of the tracking unit speed.
This output value is selectively inverted depending whether the slider moves forward or backward, is added to the output from an accelerating pulse generator to accelerate the slider, and is supplied to the slider driving coil to perform the speed control of the slider.
In a normal operation mode, the tracking loop is closed. The tracking position error is detected, and is supplied to the tracking unit, and is further supplied to the slider driving coil.
As mentioned above, since only the slider control is performed in the rough seeking mode, the force is applied to the tracking unit at the time of acceleration or deceleration, so that the tracking unit vibrates with a self-resonance frequency. Accordingly, if it is tried to strengthen the acceleration force and the deceleration force so as to move the slider at a high speed, the vibration becomes intense, so that the focus servo-control of a two axes unit becomes out of order. For this reason, the level of the tracking signal decreases, and thus the error in the track count becomes large. If the acceleration force or the deceleration force applied to the slider is further increased, the focus servo-control may become disabled. On the other hand, in case that the rough seeking is ended, when the tracking loop is being closed, there arises a problem that the drawing-in ability degrades due to the aforementioned vibration. In this manner, the unnecessary vibration of the tracking unit generated at the time of rough-seeking becomes an obstacle to accelerate the access by increasing the speed of the slider.
As a countermeasure to prevent such a vibration of the tracking unit, the tracking unit may be driven such that, for example, the force to be applied to the tracking unit is estimated from the electric current value flowing to the slider, and the force to be applied is subtracted in advance by applying the estimated value to the tracking unit (Japanese Patent Publication No. 4-35830). However, since this type of drive control apparatus for the optical pickup, indiscriminately performs the control by use of the estimated value to be applied to the tracking unit, it cannot suitably cope with the variation in the characteristics of the slider or the tracking unit.